1. Field of the Invention
This invention generally relates to piezoelectric thin-film resonator and a filter using the same.
2. Description of the Related Art
Wireless devices as represented by mobile telephones have spread rapidly, and there has been an increasing demand for a downsized and lightweight resonator and a filter equipped with the same. A dielectric substance and a surface acoustic wave have been used extensively so far; however, the piezoelectric thin-film resonator and the filter equipped with the same have excellent high frequency characteristics, can be downsized, and can be incorporated into a monolithic circuit. Therefore, the piezoelectric thin-film resonator and the filter using the same are attracting attention.
The piezoelectric thin-film resonator may be categorized into FBAR (Film Bulk Acoustic Resonator) type and SMR (Solidly Mounted Resonator) type. The FBAR type includes main components on a substrate from the top, namely, an upper electrode, a piezoelectric film, and a lower electrode. There is a cavity below the lower electrode that is overlapped with the upper electrode through the piezoelectric film. The cavity is defined by wet etching a sacrifice layer on the surface of the silicon substrate, wet or dry etching from the backside of the silicon substrate, or the like. In the present description, a membrane is defined as a film-laminated structure that is located above the cavity and a main component composed of the lower electrode, the piezoelectric film and the upper electrode. The SMR type employs an acoustic reflector instead of the cavity, the acoustic reflector being composed of films having high and low acoustic impedances alternately laminated with a film thickness of λ/4 where λ is a wavelength of an elastic wave. When a high-frequency electric signal is applied across the upper electrode and the lower electrode, an elastic wave is excited inside the piezoelectric film sandwiched between the upper electrode and the lower electrode, due to the inverse piezoelectric effect. Meanwhile, a distortion generated by the elastic wave is converted into an electric signal due to piezoelectric effect. The elastic wave is totally reflected by the surfaces of the upper and lower electrodes that respectively interface with air, and it is thus converted into a thickness-extensional wave having a main displacement in the thickness direction. In the above-mentioned structure, a resonance occurs at frequencies at which the total thickness H of the membrane is equal to integer multiples (n times) of half the wavelength of the elastic wave. When the propagation velocity, which depends on materials, is denoted as V, the resonance frequency F is described as F=nV/2H. The resonator and the filter having desired frequency characteristics can be produced by utilizing the resonance and controlling the resonance frequency with the film thickness.
Materials for the electrodes may, for example, be aluminum (Al), copper (Cu), molybdic (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Ru), or iridium (Ir). Materials for piezoelectric films may, for example, be aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), or lead titanate (PbTiO3). The substrate may be made of silicon, glass, or the like.
However, in addition to the thickness-extensional wave, the above-mentioned piezoelectric thin-film resonator has undesired waves of the lateral mode that are propagated in parallel with the electrode surface, and are reflected by the interfaces or an edge of the cavity. This adversely generates an unnecessary spurious component in the impedance characteristics of the resonator, or a ripple in the passband of the filter. This causes a problem in an application. In order to suppress such adverse affects caused by the lateral mode wave, U.S. Pat. No. 6,150,703 (hereinafter referred to as Document 1) and U.S. Pat. No. 6,215,375 (hereinafter referred to as Document 2) disclose piezoelectric thin-film resonators having electrodes including non-square and irregular polygons in which any two sides are not parallel. In the proposed piezoelectric thin-film resonators, the lateral mode waves reflected by any points are reflected and travel in different directions from the previous directions. Thus, the lateral mode waves do not resonate, so that the above-mentioned problem can be solved effectively. In addition, in order to solve a similar problem, Japanese Patent Application Publication No. 2003-133892 (hereinafter referred to as Document 3) discloses a piezoelectric thin-film resonator having an upper electrode of elliptical shape. The upper electrode satisfies 1.9<a/b<5.0, where a is the main axis of the elliptical shape, and b is the sub axis thereof.
The structures and configurations of Documents 1, 2 and 3 are certainly effective in solving the above-mentioned problems. However, the proposed structures and configurations degrade the strength of the membrane or the productivity of the cavity to the contrary. This will be described below. The thickness of the membrane, which depends on the sound speed of the material, is as very thin as approximately 0.5 to 3 μm in a wireless system having a frequency range of 900 MHz to 5 GHz. An unexpected external force easily damages the membrane, and it is thus important to consider the technique to improve the strength.
One solution is to reduce the damage of the membrane caused by internal stress by reducing the internal stress of each film at the time of forming the film. However, the inventors' study shows that piezoelectricity is improved when compression stress is exerted on the piezoelectric film, and a resonance characteristic having a large electromechanical coupling coefficient (K2) is obtainable. From this viewpoint, the membrane having compression stress is very effective if a technique to achieve a desired strength of the membrane is available. One of the effective methods is to design the membrane so that stress is evenly applied to the membrane or the membrane is not damaged easily by the same internal stress. Unfortunately, any one of Documents 1, 2, and 3 has a structurally unbalanced symmetry, and the force applied to the membrane is not equal. Thus, the membrane is easily distorted and damaged. This results in a serious problem that resonance characteristics and filter characteristics show large irregularity.
Preferably, the cavity has the same shape as that of the region in which the upper electrode overlaps with the lower electrode, and has a similar size to that of the region. If the size of the cavity is much bigger than that of the overlapping region, the membrane will be easily damaged. Thus, it is not recommended. In addition, the productivity of the cavities disclosed in Documents 1 through 3 is not good. The cavities described in Documents 1 and 2 have corners. The cavity described in Document 3 has an elliptical shape with a ratio a/b as large as 1.9<a/b<5.0 where the length of the main axis is denoted as a and that of the sub axis is denoted as b. That is, the desired shape of the cavity is not obtainable because the etching velocity is low at the corners of the cavity. The lower electrode disclosed in Document 3 has a considerably large size, as compared to that of the upper electrode. This results in stray capacitance between the overlapping extensions of the upper electrode and the lower electrode, and degrades the electromechanical coupling coefficient (K2).